Widely used nowadays as silver salt photographic systems using silver halide photosensitive materials are black and white photographic systems in which silver halide grains are reduced by a developing agent to provide developed silver for forming an image, and color photographic systems in which an aromatic primary amine developing agent is converted to an oxidized form by a development reaction, and a dye is formed by the coupling reaction of the oxidized developing agent with a color coupler is utilized for forming a dye image. However, the silver halide photosensitive materials need a particular processor and careful control for processing, resulting in limited facilities for use. The reasons are as follows: (i) processing solutions used for color developing, bleaching and fixing in the color processing steps should be precisely controlled with respect to the composition and temperature, thereby requiring professional knowledge and technical skills; (ii) these processing solutions contain chemical substances, such as a color-developing agent and an iron chelate compound as a bleaching agent, whose drainage needs regulations from the viewpoint of environment protection, whereby a developing apparatus should be accompanied by a special facility for disposing them; and (iii) despite the recent development of technology contributing to reducing the processing time, the processing of the silver halide photosensitive materials still takes a long time, failing to achieve sufficiently rapid image forming.
A thermal development photographic system in which an image is formed by heating is known as a photographic system using no processing solution comprising a color-developing agent. Heat-developable photosensitive materials utilizing thermal development for image forming and image-forming methods using them are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075; D. Klosterboer, “Thermally Processed Silver Systems”; J. Sturge, V. Walworth; and A. Shepp, “Imaging Processes and Materials”; Neblette, Edition 8, Chapter 9, pages 279, 1989, etc. The heat-developable photosensitive material generally comprises a non-photosensitive silver source that can be reduced in development, such as an organic silver salt, a catalytic amount of a photocatalyst such as a silver halide, and a silver-reducing agent, each of which is dispersed in an organic binder matrix. Such a heat-developable photosensitive material is stable at room temperature, and forms a silver image by a redox reaction between an oxidant for the silver source and the reducing agent, when it is heated at such a high temperature as 80° C. or more after exposure. The redox reaction is accelerated by catalytic reactivity of a latent image, which is formed by exposure. The silver formed in the exposed region turns black, making a contrast with the unexposed region thereby forming image.
There have been many proposals regarding methods for forming a color image by thermal development. For example, U.S. Pat. No. 4,559,290 discloses a method in which an oxidation product with no dye-releasing ability formed from a dye-releasing redox compound (DRR compound) is used with a reducing agent or a precursor thereof, the reducing agent being oxidized by thermal development depending upon an exposure amount of the silver halide, and the oxidation product of the DRR compound being reduced by the remaining un-oxidized reducing agent to release a diffusible dye. European Laid-Open Patent Application No. 220746A and Kokaigiho (Journal of Technical Disclosure), No. 87-6199, Vol. 12, No. 22 disclose heat-developable color photosensitive materials using a compound releasing a diffusible dye by the reductive cleavage of N-X bond in which X is an oxygen atom, a nitrogen atom or a sulfur atom, in place of the oxidation product of the DRR compound.
The well-known heat-developable color photosensitive material 110 utilizes a coupling reaction between a coupler and an oxidation product of a developing agent. These materials are disclosed in U.S. Pat. Nos. 3,761,270 and 4,021,240, Japanese Patent Laid-Open Nos. 59-231539 and 60-128438, etc. Generally, the coupler absorbs no visible light before color processing. Therefore, such heat-developable color photosensitive materials utilizing the coupling reaction have advantages in sensitivity as gin compared with the photosensitive materials using the above-mentioned DRR compound, so that they can be used not only as a printing material, but also as a photosensitive material for shooting.
Japanese Patent Laid-Open No. 10-260518 discloses color photosensitive materials for shooting utilizing a coupling reaction. These color photosensitive materials are attached to a processing element having a base precursor coated on a support using a small amount of water, and then subjected to heat development. However, the color photosensitive material requires the processing element and water, resulting in disadvantage in the reduction in size and simplification of the processor. Thus, expectations are high for complete dry-processing type, photosensitive materials for shooting, which are used without the processing element and water.
Japanese Patent Laid-Open No. 2000-171961 discloses complete dry-processing, mono-sheet-type, heat-developable, color-photosensitive materials and an image-forming method using such materials. However, in the case in which a photosensitive layer comprising gelatin, an organic silver compound, silver halide, a developing agent, etc. is coated on a PET base and heat-developed at 140° C. for 10 seconds in accordance with the method, the resultant image is high in haze and poor in granularity if a normal scanner is used.
Thus, the above conventional thermal developing methods fail to achieve both of sufficiently high sensitivity for shooting and low density and turbidity suitable for rapidly reading image with high quality by a scanner after developing.